ASSIGNMENT OF ELECTRONIC-TRANSITION MOMENT DIRECTIONS OF ADENINE FROMLINEAR DICHROISM MEASUREMENTS

The electronic spectra of a series of adenine derivatives have been in vestigated with respect to the number of electronic transitions, their intensities, and transition moments. The experimental work includes l inear dichroism (LD) measurements on samples partially oriented in str etched polymer poly(vinyl alcohol) films, fluorescence anisotropy (FA) , and: magnetic circular dichroism (MCD). The UV spectra of both 7-met hyladenine (7MA) and 9-methyladenine (9MA) are resolved into contribut ions from five pi --> pi transitions (I-V). Their polarizations relat ive to the C-4-C-5 axis are for 7MA +45 degrees (I, 36 600 cm(-1)), -1 6 degrees (II, 39 500 cm(-1)), -28 degrees (III, 42 600 cm(-1)), +76 d egrees (IV 45 800 cm(-1)) and similar to-29 degrees (V, similar to 47 700 cm(-1)); and for 9MA +66 degrees (I, 36 700 cm(-1)), +19 degrees ( II, 38 820 cm(-1)), -15 degrees (III, 43 400 cm(-1)), -21 degrees (IV, 46 800 cm(-1)), and similar to-64 degrees (V, similar to 48 320 cm(-1 )). The experimental results are correlated with results from semiempi rical INDO/S and ab initio CIS/6-31G(d) and CASPT2 molecular orbital c alculations. The transition moments are found to be conserved when the adenine amino group has been substituted with either one or two methy l groups. In addition, LD and MCD spectra for 6-(dimethylamino)-9-ethy lpurine, which is soluble in nonpolar solvents, have been measured in stretched polyethylene film and cyclohexane, respectively. The results indicate that the electronic transition moments of the 9-substituted adenine chromophore are essentially the same in a polar and a nonpolar solvent. On the basis of the. results,for 7MA and 9MA, the reduced LD and absorption spectra of adenine are analyzed in terms of contributi ons from the 7-H and 9-H tautomers of adenine. By comparison with theo retical and experimental results for purine and 2-aminopurine regardin g the position of the lowest n --> pi transition, we are able to conf idently position the lowest n --> pi transition in 9MA very close to the lowest pi --> pi transition. The proximity of the first (1)n pi* and (1) pi pi states in adenine might be related to the efficient non radiative deactivation of the lowest excited (1) pi pi state.